Rainfall during the wet season is sometimes supplemented by the 
passage of tropical cyclones (tropical storms and hurricanes), which most 
commonly occur from August through October. During the period 1932-1982, 
five tropical storms and eight hurricanes passed within 75 kilometers of 
Tampa Bay (Wooten 1982). Gentry (1974) reported that 5-10 inches of 
rain are usually recorded at any one point during the passage of a 
tropical storm. 
During November through May rainfall is considerably less than in 
the summer wet season. In contrast to the summer’s convective thunder¬ 
storms, rainfall during this five to six month dry season is associated 
with the passage of large frontal air masses over the state. Generally, 
winter cold fronts proceed in a southerly to southeasterly direction and 
create a preceding band of rainfall which extends along a northeast- 
southwest axis. Rainfall events associated with the passage of frontal 
systems are generally of longer duration but much less intensity than 
summer thunderstorms. These cold front rains are most common during 
January to March, creating a brief elevation in dry season rainfall. The 
driest periods of the year are normally November and April or May, as 
these months occur between periods of intense convective and frontal 
activity. 
Solar radiation varies little geographically, with a daily average 
value of 444 langleys. Highest values occur in spring rather than near 
the summer solstice due to increased cloud cover and precipitation 
(Figure 4). Correspondingly, relative humidity is normally lowest in the 
spring. Evapotranspiration varies spatially throughout west central 
Florida; estimates vary from 30 to 48 inches per year. Based on pan 
evaporation data, average yearly evaporation from open water bodies in 
the region is between 48 and 52 inches, which is only slightly less than 
the average annual rainfall. Pan evaporation rates are highest in the 
spring (Figure 4). 
Figure 4. Average monthly ventilated pan evaporation and solar radiation 
in the eastern Tampa Bay watershed (reprinted from Drew et al. 
in review). 
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